[fsp] Implement storage::WatcherManager for FSP.

chrome/browser/chromeos/file_system_provider/watcher.h

Index: chrome/browser/chromeos/file_system_provider/watcher.h
diff --git a/chrome/browser/chromeos/file_system_provider/watcher.h b/chrome/browser/chromeos/file_system_provider/watcher.h
index e073788bba2acce84d81dbbfb5adea7843d1be51..2964c97cad5b2e6f7b42bb4044b3d26325e2c1f2 100644
--- a/chrome/browser/chromeos/file_system_provider/watcher.h
+++ b/chrome/browser/chromeos/file_system_provider/watcher.h
@@ -9,7 +9,9 @@
 #include <set>
 #include <string>
 
+#include "base/callback.h"
 #include "base/files/file_path.h"
+#include "storage/browser/fileapi/watcher_manager.h"
 #include "url/gurl.h"
 
 namespace chromeos {
@@ -49,6 +51,12 @@ struct Subscriber {
 
   // Whether the subscriber should be restored after shutdown or not.
   bool persistent;
+
+  // Callback to be called for each watcher notification. It's optional, but
+  // not allowed for persistent watchers. In case of persistent subscribers,
+  // the notification should be handled using observers, as the callback can't
+  // be restored after shutdown.
+  storage::WatcherManager::NotificationCallback notification_callback;
 };
 
 // Represents a watcher on a file system.